Charged particle beam apparatuses have many functions in a plurality of industrial fields, in particular in fields having a high demand for structuring and inspecting specimens within the micrometer and nanometer scale. Typical applications of charged particle beam apparatuses include inspection of semiconductor apparatuses during manufacturing, exposure systems for lithography, detecting apparatuses and testing systems. Charged particle beams offer superior spatial resolution compared to e.g. photon beams, due to their short wavelengths.
An important factor limiting the spatial resolution of charged particle beam apparatuses is the chromatic aberration of the charged particle optical system. The chromatic aberration can be reduced, e.g., by reducing the energy spread of the charged particle beam. To reduce the energy spread, very expensive and complicated monochromators have been designed. These monochromators have the further disadvantage of reducing the beam intensity considerably.
Beyond that, a promising candidate for further improvement of charged particle beam apparatuses are charged particle beam apparatuses including a gas field ion source. Thereby, a very high resolution is aimed to be achieved due to their high brightness, small diffraction and small energy width. Experiments have shown that an energy width of <0.5 eV can be achieved for gas field ion sources having an unperturbed monocrystalline structure. Additionally, the use of a superimposed tip structure for further increasing the source brightness and further reducing the virtual source size have recently been proposed.
However, it has been found that the excellent energy width of the ion beam emitted from a gas field ion source, as described above, is deteriorated when using superimposed tip protrusions. Additionally or alternatively, the existence of different mechanisms of ionization and/or scattering might influence the energy width of the ion beam emitted by a gas field ion source. The increase in energy width can destroy the good resolution, which correlates to the energy width, as described above.
Surprisingly, it has been found that a reduction of an energy width of an ion beam is desired for gas field ion sources which are commonly known for their excellent energy width.